In a new support article published on its website, that details what do if a macOS software installer says it can’t be verified or was signed with a certificate that has expired, Apple simply says “Please download a new copy of the installer.”
When a macOS installer is damaged, you’ll see an alert that says the application can’t be verified and may have been corrupted or tampered with during downloading. In some cases, it may say that the package was signed with a certificate that has expired and may not be authentic:
“You should download the installer again. If the new installer is also affected, please contact the maker of the software for help.”
Apple has also shared links to the installation guides for its most recent macOS versions that also contain their respective download links, in case you experience the above issue with a macOS installer.